Optical Coherence Tomography (OCT) is a medical imaging technique providing depth-resolved information with high axial resolution by means of a broadband light source and an interferometric detection system. OCT has found plenty of applications, ranging from ophthalmology and cardiology to gynecology and in-vitro high-resolution studies of biological tissues.
Traditionally, axial information in OCT is obtained through interferometric methods. Time Domain Optical Coherence Tomography (TD-OCT) utilizes a variable path length in the interferometric detection system that changes in time. Thus, one of the elements in a TD-OCT system may be a variable delay line, which may be used to perform the depth scan (or axial scan) inside the sample. Several publications have described implementations of delay lines that are able to provide the necessary delay variation range at high scan speeds for their use in OCT.
For example, WIPO Patent Application Publication No. 2013/001032 A1, which is incorporated by reference herein in its entirety, describes a proposed multiplexing scheme that spreads the light into paths with different lengths using a modulator in at least one of the paths so as to separate them in frequency channels. In this way, the axial scanning distance is increased, avoiding the subsequent scanning increase in the variable delay component.
As occurs with other implementations, such as mechanical or electro-optical delay lines, the bandwidth of the system may restrict the OCT performance in terms of scanning speed. Both phase and amplitude modulation at frequencies close to the bandwidth edges elicit nonlinear behavior. In the particular case of thermo-optical modulators, phase modulation at higher frequencies generates a non-uniform optical phase response along the temperature variation. As a result, the frequency response will experience a broadening.
This broadening of the frequency response complicates the filtering process because adjacent channels will be filtered as well—generating artifacts and double images. In addition, detection bandwidth must be increased in order to recover all scanning information. Unfortunately, noise increases as the detection bandwidth increases and, therefore, the image quality or signal to noise ratio (SNR) decreases.
Working with a linear (but lower modulation) frequency regime complicates the spectrum channel separation, as the filter's order must be high. Scaling the whole system so that the delay line frequency and the modulator's frequency are reduced yields a poor frame rate performance. Although significant improvements have taken place in OCT instrumentation during the last decade, efforts have been focused on imaging speed and quality, and the progress in reduction of cost, size and complexity of systems has been merely incremental. This is believed to be one of the main factors preventing a wider adoption of OCT in emerging clinical applications beyond the well-established one of ophthalmology. Further miniaturization of OCT imaging engines has the potential to promote widespread adoption of the technique and to open a new range of applications.